Nowadays, images of a scanned objects, such as paper books or magazines, can be acquired through a device such as a scanner or a digital camera so that the scanned object is digitized. However, due to the impact of a ray of light and the bending of the surface itself of the scanned object, such as a book, the acquired images generally have shadings therein, as shown in FIG. 1. Therefore, to improve the imaging effect, it is desired to remove these shadings automatically.